Thinly slice the sirloin steak, onions, and peppers.

Heat the olive oil in a large skillet over medium-high heat and add the sliced steak.  Season with salt, pepper, and dried oregano.  Cook until the steak is cooked through and seared all over about 10 minutes.  Remove the steak from the pan and add the sliced peppers and onions.  Cook over medium-high heat for about 7-8 minutes until the peppers and onions are soft and slightly caramelized.  Add the beef back to the pan and stir everything together.

Meanwhile, heat another skillet over medium heat.

Butter one side of the tortillas (the side that will go down in the pan).  Layer half of the tortilla with pepper jack cheese slices, the meat and peppers mixture, and shredded cheese on the top.   Fold the other half of the tortilla over.  Cook the tortillas in the heated skillet until the cheese is gooey and melted, about 4-5 minutes on each side.